1. Field of the Invention
This invention relates to clock signals and more particularly to switching between clock signals.
2. Description of the Related Art
In clocking systems, a clock buffer circuit may receive several input clocks and supply a selected one of those clocks to additional circuitry in the system. Sometimes it is desirable to switch between input clocks. The desire to switch may be due to one of the clocks failing or the system requiring a different frequency of operation. When switching between clocks, a glitchless switch relies on both inputs toggling in order to perform the switch without glitches. When one of the input clocks stops or nearly stops, the switching circuit can get stuck as explained further herein. A loss-of-signal (LOS) detector is typically used to free the stuck switch. The LOS detector detects when one of the clocks has stopped toggling or toggles below a threshold rate. There are drawbacks to using the LOS approach. First, the LOS circuit has some cost to it which includes physical area. Secondly, LOS detection is a lagging indicator of the input clock's situation, which can cause the switching to not be glitchless, particularly upon exiting LOS. Third, an LOS detection circuit typically utilizes a threshold frequency or trip threshold, below which, LOS will be declared even though the clock is running For frequencies from DC to the threshold frequency, the clock switch will not be glitchless. Finally, circuits using LOS typically require a continuously running system clock. That system clock may contribute to increased output clock noise and jitter. Thus, there is a need to address the shortcomings of utilization of a LOS circuit.